1. Field of the Disclosure
The present disclosure relates generally to a structure of a vehicle body of a vehicle, such as an automobile. More specifically, the present disclosure relates to a structure of a vehicle body provided with airflow passages configured to generate an airflow in a vehicle-body underfloor space in order to reduce lift (lift effect or lift force) exerted on the vehicle body of a vehicle in motion.
2. Description of Related Art
Vehicle bodies of vehicles, such as automobiles, are usually designed to increase the flow rate of airflow in a vehicle-body underfloor space while a vehicle is travelling straight thereby reducing a lift effect exerted on a vehicle body. However, if a vehicle body is designed in consideration of only an airflow in a vehicle-body underfloor space flowing in the travelling direction of a vehicle that is travelling straight, the following problem may occur. When a crosswind impinges on the vehicle body and thus the yaw angle (the angle between the front-rear direction of the vehicle body and the direction of a resultant wind of a crosswind impinging on the vehicle body and a wind flowing in the front-rear direction of the vehicle body) increases, the flow rate of airflow in the vehicle-body underfloor space decreases. As a result, lift CL (non-dimensional force generated on the vehicle body) may increase. Especially, if a low-resistance vehicle is not designed to allow the air in a vehicle-body underfloor space to easily flow therethrough in the lateral direction of a vehicle body while the vehicle is travelling straight, the flow rate of airflow in the vehicle-body underfloor space may significantly decrease and thus lift may significantly increase. As lift CL (i.e., lift force) exerted on the vehicle body increases, the cornering force produced by a vehicle tire decreases, so that the steering performance is lowered. When the motion of the vehicle body is to be controlled using the cornering force produced by the vehicle tire through the control of, for example, steering, braking force, and driving force, a decrease in the cornering force lowers the performance (effect) of the control on the motion of the vehicle .
In view of this, there have been proposed structures for ensuring a sufficient flow rate of airflow in a vehicle-body underfloor space in order to reduce lift exerted on a vehicle body even when a crosswind impinges on the vehicle body. For example, Japanese Utility Model Application Publication No. 04-98689 describes a vehicle body structure in which a side air dam is fitted to each rocker panel located at a lower side portion of a vehicle body, at a position between front and rear wheels. This vehicle body structure is proposed in order to prevent an increase in lift due to a decrease in the flow rate of airflow in a vehicle-body underfloor space when a crosswind impinges on the vehicle body. The side air dam has a plurality of crosswind vent holes that extend from a vehicle-body side surface toward the vehicle-body underfloor space so as to be inclined toward the rear of the vehicle body.
In this structure, on both sides of the vehicle body, the extending direction in which each crosswind vent hole extends from the vehicle-body side surface toward the vehicle-body underfloor space is inclined toward the rear of the vehicle body. In this case, on the windward side, the extending direction of the crosswind vent holes substantially coincides with the direction of a resultant wind of a crosswind and a travel wind (force exerted on the vehicle in a direction opposite to the relative motion of the vehicle with respect to the surrounding air). Thus, on the windward side, an airflow easily enters the vehicle-body underfloor space from the vehicle-body side surface on the windward side through the crosswind vent holes. However, on the opposite side of the vehicle body from the side on which the air flow enters the vehicle-body underfloor space, that is, on the vehicle-body side surface on the leeward side, the extending direction of the crosswind vent holes differs from the direction of an airflow entering the vehicle-body underfloor space. Thus, the airflow that has entered the vehicle-body underfloor space collides against the wall of each crosswind vent hole located on the leeward side. In addition to such an airflow, airflows from various directions, such as an airflow from the outside of the side air dam (an airflow from the lower side of the crosswind vent holes on the windward side) and an airflow from the front side of the vehicle, enter the crosswind vent holes. Thus, the airflow easily becomes turbulent in the vehicle-body underfloor space. As a result, on the leeward side, it becomes difficult for the airflow to flow out of the vehicle-body underfloor space to the outside of the vehicle. Thus, in the vehicle-body underfloor space, the formation of an airflow from the windward side toward the leeward side in the direction of a crosswind becomes insufficient. This may cause a possibility that the effect of reducing lift using an airflow will not be sufficiently achieved.